1. Field of the Invention
This invention relates to a rear focus type zoom lens, and particularly to a rear focus type zoom lens which is used for photography, a video camera or the like and which secures a high variable power ratio and yet is short in full length and compact as a whole.
2. Related Background Art
Recently, with the tendency of home video camera and the like toward compactness and lighter weight, a striking advance has been seen in the downsizing of image pickup zoom lenses and particularly, efforts have been concentrated on the shortening of the full length, the downsizing of the diameter of the fore lens and the simplification of the construction.
A so-called rear focus type zoom lens in which the lens units other than the object side first lens unit are moved to effect focusing is known as a means for achieving these purposes.
Generally, the rear focus type zoom lens, as compared with a zoom lens in which a first lens unit is moved to effect focusing, is small in the effective diameter of the first lens unit, and it is easy to downsize the whole lens system. Also, it enables close-up photographing, particularly very close-up photographing to be accomplished and further, this is done with a relatively compact and light-weight lens unit moved and therefore, the required drive force for the lens unit may be small and quick focusing can be accomplished.
As such a rear focus type zoom lens, for example, Japanese Laid-Open Patent Application No. 62-206516, Japanese Laid-Open Patent Application No. 62-215225 and Japanese Laid-Open Patent Application No. 62-24213 disclose a zoom lens which has, in succession from the object side, a positive first lens unit, a negative second lens unit, a positive third lens unit and a positive fourth lens unit and in which the second lens unit is moved to effect a magnification change and the fluctuation of the image plane resulting from the magnification change is corrected and focusing is effected by the fourth lens unit.
Also, in single-focus lenses, for example, telephoto lenses or the like, as means for shortening the full length, there is known a lens arranged called the telephoto type in which a positive lens unit is disposed on the object side and a negative lens unit is disposed on the image plane side and the principal point position of the whole system is located on the object side to thereby earn the telephoto ratio. On the other hand, Japanese Laid-Open Patent Application No. 4-026811 and Japanese Laid-Open Patent Application No. 4-88309 propose a telephoto type in the master lens system of a zoom lens by disposing a convex lens unit on the object side of a third lens unit and disposing a concave lens in the rearmost portion, thereby shortening the full length of the zoom lens. Also, Japanese Laid-Open Patent Application No. 4-43311, Japanese Laid-Open Patent Application No. 4-153615, Japanese Laid-Open Patent Application No. 4-19165, Japanese Laid-Open Patent Application No. 5-27167 and Japanese Laid-Open Patent Application No. 5-60973 disclose examples in which a fourth lens unit is comprised of a positive lens or two positive lenses. Also, Japanese Laid-Open Patent Application No. 5-60974 discloses a zoom lens in which a fourth lens unit is comprised of two positive and negative lenses.
As described above, generally in zoom lenses, the so-called rear focus type is better suited for achieving the downsizing of the fore lens diameter and the whole system than the focusing by the movement of the first lens unit.
However, in Japanese Laid-Open Patent Application No. 4-026811 and Japanese Laid-Open Patent Application No. 4-88309, the fourth lens unit has been comprised of three lenses, i.e., a negative lens, a positive lens and a positive lens, and therefore further downsizing has been desired.
Also, in the zoom lenses disclosed in Japanese Laid-Open Patent Application No. 4-43311, Japanese Laid-Open Patent Application No. 4-153615, Japanese Laid-Open Patent Application No. 4-19165, Japanese Laid-Open Patent Application No. 5-27167 and Japanese Laid-Open Patent Application No. 5-60973, the zoom ratio is of the order of six to eight times and in zoom lenses of higher variable power than this, the fluctuation by the variable power of chromatic aberration becomes too great and it has been difficult to make the zoom lenses display a sufficient optical performance. Also, in the example disclosed in Japanese Laid-Open Patent Application No. 5-60974, both of the third lens unit and the fourth lens unit adopt an aspherical lens and therefore, for their high manufacturing costs, the zoom ratio has been of the order of eight times and sufficiently high variable power has neither been achieved.